1. Field of Invention
The present invention relates to a method of scheduling uplink resources in a cellular communication system by using Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier as the multiple access manner. More particularly, the present invention relates to a reliable adaptive resource scheduling method based on inter-cell interferences measurement for uplinks in a wireless cellular communication system so that the fluctuation of the inter-cell interferences caused by the resource scheduling may be reduced without dramatically increasing the algorithm complexity and obtain the great resource scheduling gain.
2. Description of Prior Art
As one of the key techniques in the future wireless mobile communication system, the Orthogonal Frequency Division Multiplexing (OFDM) is characterized in that the total frequency band of the system is divided into a great number of sub-carriers which are transmitted in parallel, have flat fadings, partially overlap with one another and are orthogonal with each other, whereby it efficiently protects the frequency from selective fadings, reduces the inter-symbol interferences caused by the multi-path, and increases the frequency spectrum utilization. Due to these technical advantages, the Orthogonal Frequency Division Multiplexing technique becomes one of the candidates to be used in the physical layer in the next generation wireless communication system researches such as IEEE 802 standard series and 3GPP long term evolution.
In addition to the above basic technical features, the application of the multiple access techniques based on the Orthogonal Frequency Division Multiplexing (for example, the Orthogonal Frequency Division Multiple Access (OFDMA) and DFT-S-OFDMA (a kind of single carrier which is characterized in both the low peak-average power ratio of the traditional signal carriers and the Orthogonal Frequency Division Multiplexing)) may further achieve multi-user diversity. Because different users may experience different fadings at the same time, a carrier having a severe fading for one user may make another user get a good channel quality. Thus, combining OFDMA or DFT-S-OFDMA with an efficient carrier resource scheduling method can dramatically increase the system throughput, and increase the frequency spectrum utilization.
However, in a wireless cellular communication system using OFDMA or a single carrier technique as the multiple access manner, the specific uplink inter-cell interferences will represent a characteristic of non-white noise in the frequency domain, and will fluctuate during the resource scheduling in the time domain. Such fluctuation will be troublesome for efficiently scheduling the resources. In the future wireless cellular communication system using OFDMA or DFT-S-OFDMA technique as the core technique, the interferences influencing the user equipments include inter-cell interferences and intra-cell interferences. The orthogonality among the sub-carriers or sub-channels ensures the orthogonality of the users within the same cell. Therefore, the inter-cell interferences become the main interferences on the user equipments. The inter-cell interferences represent a characteristic of non-white noise in the frequency domain. That is, the user may experience different interferences on different sub-carriers. Thus, during the resource scheduling, the frequency selectivity of the interferences on the user equipment has to be considered while considering the frequency dependent fadings on the user channel. Additionally, it is more important that the inter-cell interferences fluctuate during the resource scheduling, i.e., the resource scheduling will change the values of the inter-cell interferences, which is also a problem to be considered during the resource scheduling. Then, in the future wireless cellular communication system, the uplink resource scheduling and the inter-cell interferences will be a pair of inter-restricted factors.
In a CDMA system, all users within the same cell share the frequency resources. The interferences on user equipments and base stations within one cellular cell come from all users within other cells of the system. Generally, it is assumed that the inter-cell interferences in the CDMA system are Average White Gaussian Noise (AWGN). In a case where the frequency multiplexing factor is 1, signals sent by all the users in the adjacent cells will bring the inter-cell interferences. The overlapping of a great number of users' signals will average the difference between the interference signals of different users so that the actual inter-cell interference signals have a flat characteristic in the frequency spectrum similar to the AWGN.
Unlike the CDMA system, in an OFDM or single carrier based cellular system, within a same cellular cell, each sub-carrier or sub-channel is occupied by only one user. In this case, interferences from adjacent cells on a certain user equipment or base station only include those caused by the users using the same carrier or channel in other cells. That is, in the OFDM system, the interferences from the adjacent cells are much less. Such less interferences will result in dramatic changes in the inter-cell interferences during the resource adjustment of the corresponding cells. There is only one interfering user within one cell, and thus the inter-cell interferences may not have a behavior similar to the AWGN.
Firstly, the interference signals are subjected to the multi-path transmission and thus represent frequency dependent fadings. Secondly, the interference signals on different sub-carriers may also come from the different users within the adjacent cells. These users' signals may be subjected to different path losses, shadow fadings and channel fast fadings so that the strengths of the interference signals are not uniformly distributed in the frequency spectrum. Additionally, since there is no overlapping of multiple users' signals, the fluctuations in the strengths of the interference signals when the sub-carrier or sub-channel is reassigned can not be averaged. Therefore, the inter-cell interferences represent a non-white noise characteristic.
FIG. 1 shows the simulated condition of the inter-cell interferences in the OFDMA based wireless cellular communication system. When the number of the interference sources reduces to a certain degree, the interferences from the user equipments within the adjacent cells on the user equipment using the same carrier or channel with in the present cell dramatically fluctuate over time. The interference curve shown in FIG. 1 represents the non-white noise characteristic.
In addition to the characteristic of non-white noise in the frequency domain, the inter-cell interferences also change in the time domain along with the intra-cell resource scheduling. One resource allocation scheme determines one kind of non-white noise for the inter-cell interferences, and the scheduled another resource allocation scheme will determine another kind of non-white noise for the inter-cell interferences. This phenomenon is shown as that the inter-cell interferences fluctuate during the resource scheduling.
FIG. 2 shows the simulated result of the change in the inter-cell interferences on the single carrier on the user equipment during the intra-cell resource scheduling. The curve shown in FIG. 2 gives the characteristic of the fluctuation of the inter-cell interferences during the resource scheduling. The resource scheduling includes reallocation of the sub-carriers or channels, or power changes within the cell. The resource scheduling or reallocation within the adjacent cells of the communication system will result in the dramatic changes in the interferences signals in the interested cell (the present cell). That is, the interferences from the adjacent cells are different from those in-advance expected ones.
As described above, in the cells of the wireless cellular communication system using the OFDM, since the inter-cell interferences from the adjacent cells fluctuate dramatically, the algorithm in which the resource scheduling in the system is applied based on the measured Signal/Noise Ratio (SNR) is not applicable any more. On one hand, it is because the corresponding calculation can not be performed dependent upon the inaccurate Signal/Noise Ratio; and on the other hand, the adaptive encoding and modulation will be error since the adaptive encoding and modulation has a very restricted requirement to the stability of the Signal/Noise Ratio. If the estimation of the Signal/Noise Ratio has some deviations, the mechanism of the adaptive encoding and modulation will be greatly influenced, which will result in the errors in the allocated encoding rate and modulation order.
Due to the characteristic of non-white noise, the resource scheduling algorithm shall consider not only the fading conditions of respective sub-carriers or single carrier, but also the inter-cell interferences on the sub-carriers or single carrier. Unlike predicting the channel fadings by channel estimation, however, the characteristic of fluctuations during the resource scheduling makes the inter-cell interferences be burst and unpredictable during the resource scheduling. Therefore, if the inter-cell interferences are obtained dependent on the estimation before scheduling, as is done in traditional resource scheduling, the user equipments within the adjacent cells after the resource scheduling will experience quite different interferences and thus the system performance will be greatly reduced.
FIG. 3(a) shows the inter-cell interferences estimated by the user before the resource scheduling by stimulating the prior art, and FIG. 3(b) shows the actual experienced inter-cell interferences after the scheduling. From FIGS. 3(a) and 3(b), it can be seen that the main problems in such scheduling are as follows.    1. On a sub-channel whose interferences are estimated over-high, the resource can not be completely used;    2. On a sub-channel whose interferences are estimated over-low, the user equipment will experience worse channel quality;    3. The deviation of the interference estimation will result in the erroneous estimation of the signal interference noise ratio and further influence the exactness of the adaptive encoding and modulation.
Therefore, in the next generation wireless cellular system using the OFDMA or single carrier technique as the multiple access technique, the characteristics of non-white noise and fluctuation during the resource scheduling of the inter-cell interferences require that the resource scheduling algorithm used in such system shall consider the coordinate among the cells in order to efficiently reduce the negative influences caused by the inter-cell interferences.
A plurality of algorithms for reducing the inter-cell interferences, including interference elimination, interference randomization and interferences coordinate, are introduced in Dynamic Resource Allocation With Buffer Constraints In Broadband OFDMA Networks, Guoqing and Hui Liu, WCNC′ 2003, March, 2003, New Orleans (Reference 1). However, these methods only address how to reduce the inter-cell interferences, but cannot efficiently solve the conflicting between the fluctuation of the inter-cell interferences and the resource scheduling. An integrated resource scheduling method is introduced in Carrier Assignment Algorithms In Wireless Broadband Networks With Channel Adaptation, Iordanis Koutsopoulos and Leandros Tassiulas, Proc of ICC. 2001, vol. 5, pp. 1401-1405 (Reference 2), which may mitigate the problems caused by the above algorithms only considering the reduction of the inter-cell interferences. However, the algorithm proposed in Reference 2 has a higher complexity since it requires to estimate the intra-cell and the inter-cell channels, and thus has a higher overhead, which limits its application in the actual system.